Dressed in their best tuxedos and a distinctive ring of white eyeliner, Adélie penguins return to Cape Adare, Antarctica, every breeding season – October to February – to raise a family after spending the winter foraging at the edge of the fast ice growing out from the shore. Pairs often reuse their rock pile nest from previous seasons, and travel thousands of miles to return to the exact same spot every year. A few quick renovations, like building up walls knocked over by gusts of wind, and replacing pebbles pickpocketed by neighbors, and they’re ready to go. The colonies are noisy, and prime real estate near the water is at a premium with hundreds of thousands of breeding pairs staking claims.

The best sites are on Ridley Beach, a 0.31 square mile section of Cape Adare that juts out into the Southern Ocean. It’s covered in smooth, black pebbles, sculpted into ridges by the relentless, heavy surf. Pools of standing seawater break up the landscape, trapped ashore by storm surges and heavy winds. During breeding season, it’s hard to tell exactly what the ground looks like at Ridley Beach because every inch of it is covered by penguins.

There used to be even more of them, reports Steven Emslie, a marine ornithologist based at the University of North Carolina. His survey of active and abandoned penguin settlements on Cape Adare, recently published in Royal Society Open Science, details the rise and fall of the world’s largest Adélie penguin colony over the last 2,000 years. It’s the end result of two field expeditions, in January 2005 and 2016, to the bottom of the world.

The Adélie penguin empire in the Ross Sea is dynamic, and has rolled with the punches of shifts in sea ice and climate change for millennia. Knowing the details of this natural history is a useful tool for predicting how populations may fare in an uncertain future, as warming temperatures and rising sea levels steal access to proven breeding sites.

Picking their way between nests, chicks, and cranky parents, interloper scientists searched for clues to reconstruct the natural history of penguins in Antarctica. Like bygone human civilizations, information about ancient penguin settlements here is written in what’s been left behind – old feathers, bones, and eggshells. Armed with a handheld GPS, a trowel, and a tarp, the researchers systematically excavated old, unused pebble mounds, looking for buried treasure to bring back to the lab for analysis.

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Back in the lab, radiocarbon dating analysis aged each of their samples. By comparing the amount of carbon-14, which decays over time at a known rate, with the amount of stable carbon-12 in a sample, researchers can precisely estimate the age of a material.

They found that penguins have been on Cape Adare for a very long time. While Cleopatra and Julius Caesar were fighting civil wars, Adélie penguins were waging a campaign of their own far to the south. Driven out of colonies along the Scott Coast by encroaching ice cover during a cooling period in Antarctic history, they scoured the coast for polynyas, areas of open water surrounded by sea ice, that make perfect nurseries. Adélie penguins need reliable access to the sea to forage for food, and spend the vast majority of their lives by the water. Large-scale changes in ice formation forced them to find new, warmer breeding sites. The first penguin pioneers arrived at Ridley Beach about 50 CE, roughly 150 generations removed from its current inhabitants.

Bolstered by huge swarms of krill and ocean upwelling, the colony ballooned in size over the next 800 years. Penguins occupied not only the entire Ridley Beach, but most of the upper terrace overlooking it, a lengthy, treacherous walk from the sea. Even penguins will suffer a rough commute if it means they can have a nice, safe place to raise their children. At its height, this penguin boomtown was a supercolony composed of over 500,000 breeding pairs.

But few penguins live on the steep slopes of the terrace anymore. Sometime after the eighth century, the colony mysteriously collapsed to its present size of about 340,000 breeding pairs. Perhaps the wind patterns changed, or the air temperature cooled, or the stretch of open water near the beach shrunk in size. Similar events have caused total breeding failure, lower chick numbers, and increased mortality in other colonies along the Ross Sea. Antarctica is an unpredictable land of extremes, and it’s anybody’s guess what actually happened.

Ridley Beach remains fully occupied today, but is increasingly swamped by storm surges and waves whipped up by high winds. Melting glaciers and climate changes are causing the sea level rise quickly in the area, drowning out more and more waterfront nest property every year. In the near future, the low-laying areas occupied by colony will be in danger of submersion, making the beach uninhabitable.

The Adélie penguin empire has rolled with the punches of shifts in sea ice and climate change for millennia

Where will the penguins go? We can look at their history for answers. Cape Adare penguins will probably retreat to the higher ground of the upper terrace, as they did before when the beach was too crowded. But not all colonies along the Ross Sea have penguin-friendly slopes near the shore, and the effects of climate change will soon displace hundreds of thousands of penguins from their historic breeding grounds. Like their ancestors 2,000 years ago, they’ll have to set off into unknown waters and establish colonies elsewhere. Maybe they’ll be able take up residence in old breeding sites once cut off by ice formation, but now accessible due to glacial melting and ice breakout. Perhaps they’ll go elsewhere.

With the breakneck speed of climate change, accelerated by human activities, it’s unclear if these waddling, curious, and obstinate birds will be able to adapt.

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I study seabird foraging, so this article about penguins really appealed to me! As soon as I read the part about a colony collapse, I wondered if it was related to foraging? Adelie penguins eat primarily krill, euphausids, and fish, depending on the colony. This original paper found that this Cape Adare colony eat more krill than fish, compared to other colonies in the Ross Sea region.

It seemed possible that maybe a decline in krill would cause this colony to collapse, but this paper reports plenty of food are available for the penguins. So, probably not a foraging issue.

Then I wondered about energetics. If this Ridley Beach colony has to move upslope because sea level rise makes their current low nesting site uninhabitable, it may be more energetically expensive for these penguins. They may spend more time walking and standing at the upper terrace site than at the Ridley Beach site, and it might take more effort to get to the water to find food. For example, it would take me a lot more time and energy to have to hike somewhere to find my food than if I had food readily available in my backyard. Environment affects these penguins so much that I guess only time will tell.

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